The present invention relates to a protection circuit for switches, in particular such as those which are used in switched-mode power supplies, and to a switched-mode power supply relating to this.
Switches with a high-impedance control input, such as MOSFETs for example, are increasingly being used in switched-mode power supplies. The high impedance of the control input means that the switched-mode power supply can enter an undefined state in the event of a defect, which state can destroy the switch. This is possible, for example, in the event of a short circuit at the output of the switched-mode power supply or in the event of an interruption in the switched-mode power supply control loop, resulting in an interruption in the closed-loop-controlled switch operation. However, in cases such as this, a residual voltage may still be present at the control input of the switch, in the case of switched-mode power supplies, due, in particular, to their starting circuit, so that the switch is partially actuated. If the switch is only partially actuated, a high current flows through it when a voltage is still at the same time applied to it, so that its power loss becomes excessive and, in consequence, it is destroyed.
DE 197 35 208 A1 discloses a free-running switched-mode power supply having a protection circuit which reliably switches the switching transistor off when a short circuit occurs. In one exemplary embodiment, the control voltage at the input of the switching transistor is dissipated through a transistor stage, so that the switching transistor reliably remains switched off in the event of a short circuit.
The object of the present invention is to specify a protection circuit and a switched-mode power supply relating to this for a switch, which protection circuit prevents the switch from being actuated, and thus prevents it from being destroyed, in the event of a fault.
The protection circuit of the present invention contains an energy-storage means which is connected to the control input of the switch and is charged with a delay via resistance means when a control voltage is present, and is then discharged via switching means when the control voltage is absent or low. Since switches of this type are normally actuated by square-wave voltages which are, for example, pulse-width modulated or whose frequency is variable for control of a switched-mode power supply, the energy-storage means is in this way charged slowly during the on-signal for the switch, but is discharged once again in the same way during the xe2x80x9coffxe2x80x9d signal. It is thus impossible for a higher voltage to build up across the energy-storage means during normal switch operation.
The protection circuit also contains a switching stage which is connected to a supply voltage for the switch, switches on when there is a specific threshold voltage across the energy-storage means, and in consequence reduces the supply voltage. The energy-storage means is, for example, a capacitor, and the switching stage is a single transistor stage or a pair of transistors connected in the form of a thyristor circuit.
When the switching stage is switched on at the said threshold voltage, the energy-storage means is at the same time discharged via the switching stage. A portion of the charge dissipated from the supply voltage is in this case at least partially routed to the energy-storage means, so that the phase during which the switching stage is switched on is lengthened. This is achieved by means of a diode in parallel with the energy-storage means. This diode at the same time prevents inverse charging of the energy-storage means.
The protection circuit can be used, in particular, for switched-mode power supplies such as those used in television sets or video recorders, for example. Switched-mode power supplies of this type normally have a supply voltage which is fed from a secondary winding of the transformer, and a starting circuit which supplies a supply voltage once the switched-mode power supply is switched on, until the switched-mode power supply supplies itself via the supply stage.
A control voltage is applied to the control input of the switch, by means of which the switched-mode power supply stabilizes output voltages via a control circuit arranged either on the secondary side or the primary side. The control input is decoupled from the starting circuit and from the supply stage via a resistance means. The energy-storage means in the protection circuit is in this case connected to the control input of the switching transistor, and the switching stage is connected to the starting circuit and to the supply stage.
In the event of a fault, a situation may arise in which the closed-loop-controlled switch operation is interrupted and a voltage builds up, for example due to the starting circuit, at the control input of the switch, and this voltage could at least partially switch the switch on. This is identified by the protection circuit, and the supply voltage and the voltage at the control input of the switch are reduced appropriately by the protection circuit, so that there is no risk to the switch.
A low-impedance current-measurement resistor is often connected downstream from the output of the switch in a switched-mode power supply, to supply current information for the control circuit. The output of the protection circuit is then advantageously connected to the higher-potential output of the switching transistor, rather than to earth.